Commit message: Fix EinsumHelper::ParseEquation to avoid uninitialized accesses.

EinsumHelper::ParseEquation is supposed to return true or false in
input_has_ellipsis and output_has_ellipsis to indicate whether there is
ellipsis in the inputs and output. Previously, when there is no ellipsis in the
inputs or output, the routine doesn't assign false to the variables. This
change initializes the two variables with false to fix the problem.
PiperOrigin-RevId: 391772004
Change-Id: I17b6c88aadef4131470378e48cced054bf252e86
CVE: CVE-2021-41201
CWE: CWE-703
Vulnerability description: TensorFlow is an open source platform for machine learning. In affeced versions during execution, `EinsumHelper::ParseEquation()` is supposed to set the flags in `input_has_ellipsis` vector and `*output_has_ellipsis` boolean to indicate whether there is ellipsis in the corresponding inputs and output. However, the code only changes these flags to `true` and never assigns `false`. This results in unitialized variable access if callers assume that `EinsumHelper::ParseEquation()` always sets these flags. The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, TensorFlow 2.5.2, and TensorFlow 2.4.4, as these are also affected and still in supported range.

Project: tensorflow
File: einsum_op_impl.h

  static Status ParseEquation(const string& equation,
                              OperandLabels* input_labels,
                              Labels* output_labels,
                              std::vector<DimensionType>* label_types,
                              OperandLabelCounts* input_label_counts,
                              LabelCounts* output_label_counts,
                              gtl::InlinedVector<bool, 2>* input_has_ellipsis,
                              bool* output_has_ellipsis) {
    gtl::InlinedVector<string, 2> input_str;
    string output_str;
    TF_RETURN_IF_ERROR(ParseEinsumEquation(equation, &input_str, &output_str));

    // Temporary map from single character labels to (consecutive) integer
    // labels.
    absl::flat_hash_map<char, int> label_mapping;
    int num_inputs = input_str.size();
    input_labels->resize(num_inputs);

    // Map from single characters to integer labels.
    for (int i = 0; i < num_inputs; ++i) {
      MapToLabels(input_str[i], &input_labels->at(i), &label_mapping);
    }
    MapToLabels(output_str, output_labels, &label_mapping);

    // Compute counts for input and output labels.
    int num_labels = label_mapping.size();
    input_label_counts->resize(num_inputs);
    input_has_ellipsis->resize(num_inputs);
    for (int i = 0; i < num_inputs; ++i) {
      input_label_counts->at(i).resize(num_labels);
      for (const int label : input_labels->at(i)) {
        if (label != kEllipsisLabel)
          input_label_counts->at(i)[label] += 1;
        else
          input_has_ellipsis->at(i) = true;
      }
    }
    output_label_counts->resize(num_labels);
    for (const int label : *output_labels) {
      if (label != kEllipsisLabel)
        output_label_counts->at(label) += 1;
      else
        *output_has_ellipsis = true;
    }

    // Map each label to a unique DimensionType.
    label_types->resize(num_labels);
    for (int label = 0; label < num_labels; ++label) {
      if (label == kEllipsisLabel) continue;
      bool removed = (*output_label_counts)[label] == 0;
      bool unique = num_inputs == 1 || (*input_label_counts)[0][label] == 0 ||
                    (*input_label_counts)[1][label] == 0;
      (*label_types)[label] = GetDimensionType(removed, unique);
    }
    return Status::OK();
  }